Diode-lasers with analog-modulated output are used as illumination sources in confocal microscopy. Diode-lasers are typically driven (powered) by a variable direct current (DC) supplied by a simple voltage-to-current converter. The current is varied by varying a DC voltage applied to the converter with a linear relationship between the current and the voltage. In confocal microscopy, it is usually desired that the diode-laser power (light-output) be modulated from zero to some maximum value in a ramp or “sawtooth” wave-form.
A preferred way of achieving this modulation form is to drive the diode-laser using a comparable voltage wave-form applied to the current converter. This is complicated by the fact that a certain threshold level of voltage (and corresponding current) is required before laser-action is initiated in the diode. At a voltage less than the threshold voltage, the diode will emit light (radiation) in the form of spontaneous emission or fluorescence. The threshold voltage can be almost one-half of the voltage that produces maximum laser-light output.
This is schematically illustrated in the graph of FIG. 1 which depicts diode-laser power as a function of applied voltage. The curve is representative of the output power response to a linear voltage-ramp. It can be seen that at voltages between zero and the lasing threshold voltage, non-laser light-output increases proportionally but weakly compared with the light-output increase once laser action is established. The entire output curve is a distorted replica of the linear voltage ramp. Any other modulation waveform such a sine-wave would be correspondingly distorted.
A generally practiced method of reducing, if not altogether eliminating the distortion is to apply a fixed bias-voltage (and corresponding current) to the current converter such that, when the modulation voltage is only slightly greater than zero, the lasing threshold voltage is reached. This is schematically illustrated, graphically, in FIG. 2
Here, the bias voltage has been selected such that the lasing threshold is reached when the modulation voltage is only a fraction of a volt. This fractional voltage is selected to be only just sufficient that certain statistical variations can be accommodated. These include variations in the lasing threshold, variations in an externally provided modulation drive signal, and variations in internal signal processing circuitry. The response to the linear voltage ramp is now distorted significantly less than in the example of FIG. 1 without bias. Here, however, it should be noted that at zero modulation voltage there is still some non-laser light-output from the diode-laser. This is not acceptable in the confocal microscopy systems. Accordingly, a modulation method is needed that will provide the minimized output-distortion of the method of FIG. 2 but with zero light-output at zero modulation-voltage.